When boreholes or wells are drilled into the earth for mining purposes, for the drilling of oil and gas wells, or for any one of a wide variety of other reasons, it is generally advantageous, or in some cases critical, to survey and record the progression of the drill bit and the position of the borehole in order to monitor bore drift and the angle of inclination of the well. The availability of a method and apparatus that enables a drill operator to map the progression of the well and to monitor the angle of its inclination is particularly important where there is a desire to drill into a specific underground formation. Knowing the location of the drill bit, and knowing the angle of inclination of the well relative to vertical, can also be extremely important when drilling deep wells, and in the case of oil and gas drilling where a large number of wells are sometimes drilled in a closely spaced configuration within a confined geographic area.
To assist drillers in monitoring and logging the inclination and progression of a borehole, others have devised and proposed a wide variety of different measurement devices. Such devices are generally capable of determining the location of a portion of a borehole or a drill bit relative to its surface entry point based upon a defined coordinate system. Most commonly a Cartesian coordinate system is utilized and centered at the wellhead with the “Z” axis defined as extending from the wellhead toward the center of the earth and the “X” and “Y” axes extending in a north-south and east-west configuration, respectively. Typical devices that are currently in use for surveying or mapping a borehole comprise downhole tools or probes that contain instrument packages capable of taking measurements and sending signals to equipment at the surface that can be used to plot the position of the borehole. The instrument packages of such tools or probes commonly contain gyroscopes, magnetic compasses, pendulums, accelerometers, and devices or sensors to measure the length of the borehole from the wellhead to the downhole tool.
While such devices have been in use for a considerable length of time, they continue to suffer from a number of inherent limitations and disadvantages, one of the more significant of which is their cost. In the drilling of oil and gas wells the depletion of relatively shallow and accessible hydrocarbon deposits has resulted in the necessity to drill deeper boreholes and to target smaller geographical formations. In deep wells there is an enhanced requirement for producing an accurate survey of the borehole as even a small degree of drift can become very significant over the span of several thousand feet. For this reason, the tools and probes that have been developed and that are currently in use tend to include instrument packages having relatively sensitive sensors. Unfortunately while these sensors are designed to be reasonably accurate they tend to be susceptible to mechanical noise, which is relatively common in the downhole environment where they are required to operate.
The effects of background or mechanical noise is particularly troublesome when measuring small angles of drift or inclination. Accordingly, to offset the effect of noise and the error that noise can introduce into sensor readings, manufacturers of existing systems have resorted to using high quality (and hence expensive) sensors, in conjunction with noise filters, to help minimize the noise effect and to help to send a “cleaner” signal to the surface. Such systems are inherently more complex, present increased opportunity for malfunction or breakdown, and can significantly increase the overall cost of the tool.